D/H FRACTIONATION OF COEXISTING BIOTITE AND HORNBLENDE IN SOME GRANITIC ROCK MASSES

The deuterium content (δDSMOW) of the water extracted from coexisting biotite and hornblende in granitic rocks was measured. The relationship between δD of biotite and hornblende suggested an isotopic equilibrium in the rocks of four granitic bodies in the Kitakami mountains (Northeast Japan), and the maximum temperature of isotopic equilibration of biotite and hornblende was estimated to be 700-1, 000°C, provided that the minimum temperature was 450-550°C. The δD value obtained for the water with which these minerals equilibrated was -29_??_-37‰ and is definitely enriched in deuterium relative to the present local surface water. On the other hand, the relationship between δD of biotite and hornblende in the Ryoke metamorphic belt contiguous to the Median Dislocation Line (Southwest Japan) indicated an isotopic disequilibrium. In this case a complicated history of formation of these granitic rocks in terms of the interaction of water and hydrous silicates was inferred.